The field of the invention is systems and methods for magnetic resonance imaging (“MRI”). More particularly, the invention relates to systems and methods for reducing power deposition, sometimes quantified as the specific absorption rate (“SAR”) producing during a substantially simultaneous multi-slice acquisition.
Since its initial application, scan time for volume coverage with echo-planar imaging (“EPI”) or spiral-type MRI data acquisitions has not substantially decreased. Nearly all the successful efforts to shorten EPI acquisition times have targeted reducing the number of refocused echoes needed for spatial encoding to form an image, such as by means of partial Fourier imaging, parallel imaging, or sparse data sampling techniques. Although these approaches decrease scan time for spatial encoding of a single slice in EPI, they do not necessarily reduce the time required for image acquisitions by a significant amount.
Recently, additional accelerations have been demonstrated by “slice multiplexing,” whereby multiple image slice locations are excited and acquired simultaneously using a multiband (“MB”) radio frequency (“RF”) pulse, a technique commonly referred to as multiband imaging. The MB technique is limited in part by power deposition and SAR considerations, especially at high magnetic fields, and in part by the peak power and/or voltage that can be tolerated by the RF coil circuitry or the peak power and/or voltage that can be generated by the amplifier in an MRI system. These limitations arise because multiband RF pulses that are employed in MB imaging typically represent the sum of individual, single-band RF pulses that affect only spins in a single slice location. This sum of individual single-band RF pulses leads to an increase in the voltage of the multiband RF pulse that is linear with the number of different slice locations that are simultaneously manipulated, a quantity also referred to as the multiband factor. Because of this voltage increase, the power applied to the RF coil increases quadratically with the multiband factor when the pulse duration and, if repeated, the number an duration of such repetitions are kept the same as for a single-band RF pulse application. For the multiband technique, the multiple applications of multiband RF pulses to image an entire volume-of-interest increases the power applied to the RF coil, as well as the power deposited to the subject, quadratically with the multiband factor of the multiband RF pulses used. Again, this quadratic increase in power is relative to the power applied of deposited when the same general pulse form is employed to accomplish single versus multiple slice excitation and the time required to cover the volume-of-interest is reduced correspondingly by the multiband factor
SAR, which is a measure of the rate at which energy is absorbed by the body when exposed to an RF electromagnetic field and is measured in watts per kilogram of tissue (“W/kg”), is a concern when conducting MRI experiments on human subjects. As noted above, SAR is especially a concern during the simultaneous excitation of multiple slice locations. This is because when multiple RF pulses are simultaneously employed, the local electric fields generated by each RF pulse undergo local superposition and local extremes in electric field magnitude may arise, leading to spikes in local and global SAR that are of concern to regulatory bodies in both the United States and Europe. For a discussion of these regulatory concerns in the United States, see, for example, Center for Devices and Radiologic Health “Guidance for the Submission of Premarket Notifications for Magnetic Resonance Diagnostic Devices,” Rockville, Md.: Food and Drug Administration; 1998, and in Europe, see, for example, International Electrotechnical Commission, “International Standard, Medical Equipment-Part 2: Particular Requirements for the Safety of Magnetic Resonance Equipment for Medical Diagnosis, 2nd Revision,” Geneva: International Electrotechnical Commission; 2002.
The need to stay below safe SAR limits often requires unfavorable tradeoffs in acquisition parameters such as increased repetition time (“TR”) or reduced flip angle. SAR becomes especially problematic at field strengths of 3 T and higher, where the power needed for a given flip angle increases approximately quadratically with magnetic field magnitude; thus, an increase of as much as four-fold as compared to 1.5 T applications can be present.
In general, the use of multiband RF pulses increases the peak RF power requirements. Moreover, when the use of the multiband RF pulses is employed to shorten the TR for volume coverage relative to a single-band acquisition, while retaining the same volume coverage, the average SAR is also increased due to the faster repetition of the RF pulses. Correspondingly, if the same TR is used, but the volume coverage is extended, the average SAR is also increased. The potentially significant increase in the SAR therefore restricts the use of multiband imaging sequences to low SAR pulse sequences, such as gradient echo EPI, especially at high magnetic fields. Even in such low-power applications, however, SAR and/or the tolerance of the RF coils and the electrical components of the RF chain may run into limitations.
It would therefore be desirable to provide a method for the simultaneous manipulation of spins using RF pulses that manipulate multiple slice locations with a reduction in power deposition, which may be measured as SAR, produced by the application of the RF pulses and/or reduction in peak power an voltage applied to the RF coil. Examples of spin manipulation include excitation, inversion, and refocusing. Such a method would broaden the applicability of multiband RF pulses to imaging pulse sequences other than historically low SAR sequences, and would allow for improving other imaging parameters, such as TR and volume coverage, without risking unsafe levels of SAR.